rake test silently fails on syntax errors in tests

In this test

- (void) testCanSerializeJSONWithNulls
{
    [self assert:"null" equals:[CPString JSONFromObject: null ] message:"1"];
    [self assert:null same:[[CPString JSONFromObject: null ] objectFromJSON] message:"2"];
    [self fail:[[CPString JSONFromObject: [ null ] ]];
    [self fail:"[[CPString JSONFromObject: [null] ]"];
    [self assertFalse:true];
    [self assert: [null] same:[[CPString JSONFromObject: [null] ] objectFromJSON] message:"3"];
    [self assert: {foo:null} same:[[CPString JSONFromObject: {foo:null} ] objectFromJSON] message:"4"];
}

the line [self fail:[[CPString JSONFromObject: [ null ] ]]; makes the parser barf and the tests just don't execute - but no error is shown.